If you don't sell the JD while it still has some value you'll regret it later. Yeah, if you can't get anything for it you can call it a back-up.
But it's not. Think about it. You jump off of your new, faster, more efficient mower and get on the old JD. Then it occurs to you "Wouldn't my time be better spent getting the new mower fixed?" And probably the old JD breaks down from sitting around for months. Then you have two repair bills.

If you're worried about keeping on the job, talk to your dealer about a loaner if yours breaks.
Something I've tried with success is talking to the service manager. I told him I would pay the overtime to get my mower fixed. Plus I'd tip the mechanic $20.
The next time I showed up the service manager asked his crew "Who wants to work thru lunch on overtime pay?" They ALL did.

And like I mentioned before if you're concerned about reliability & durability pick something not sold in the big box stores. If you go to a dealer that sells to commercial cutters they have a whole different attitude about getting you back on the job. But you have to ask.

If you didn't have a back-up mower for your JD you're going to need it even less with a new machine.